1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, more specifically to an electrophotographic photosensitive member having a specific layer constitution for direct electrification. The present invention also relates to a process cartridge and an electrophotographic apparatus employing the above electrophotographic photosensitive member.
2. Relating Background Art
The electrophotographic photosensitive member having a layer composed mainly of a resin or a resin layer containing a photoconductive substance has advantages of especially high productivity, relative inexpensiveness, and characteristics controllable by selection of the employed photoconductive substance. Accordingly, this type of photosensitive member is widely used practically.
The electrification means employed generally for the electrophotographic apparatus utilizes corona discharge caused by application of a high voltage to a wire. In recent years, direct electrification means are practically used which electrify a photosensitive member by applying voltage to a roller-shaped or plate-shaped electrification means brought into contact with it, because this type of electrification means requires lower voltage application and evolves less ozone. For uniform electrification with this direct electrification means, application of pulse voltage derived by superposition of DC voltage and AC voltage has been proposed.
The direct electrification means, however, has a disadvantage that the photosensitive member tends to be abraded by friction during repeated use, particularly at the end portion of the contacting zone of the electrification means on the photosensitive member rather than the middle portion thereof. This tendency is more pronounced when DC-AC superposed pulse voltage is applied, or when the voltage between the applied voltage peaks is raised or the pulse frequency is increased to accelerate the processing speed of the electrophotographic apparatus.
The resin-containing layer of the electrophotographic photosensitive member is usually formed by dip coating. The dip-coated layer is liable to be thinner at the top end portion in the dip coating than that at the middle and the bottom portions. The electrophotographic photosensitive member thus formed is liable to be abraded more at the thinner layer portion.
In the abraded thinned portion of the layer of the photosensitive member on a support, the surface potential becomes lower, causing lower image density in normal image development or fogging in reversal image development. The greater abrasion will give rise to dielectric breakdown to cause strip-like defects of the formed image.
The present invention was accomplished based on the consideration by the inventors of the present invention that the above phenomenon is ascribable to the joule heat of the electric current through the photosensitive member on application of voltage. According to the consideration, when the electrification member is brought into contact with the photosensitive member, the contact pressure tends to be higher at the end portion of the electrification member than the middle portion thereof, which enlarges the contacting areal size at the end portion of the contacting zone to allow larger electric current to flow, thereby causing more abrasion. The AC current flows more readily than the DC current, tending to cause abrasion. The higher voltage between the voltage peaks causes larger electric current to give abrasion. A higher frequency reduces impedance of the circuit to cause a larger current to flow to give abrasion. A smaller thickness of the layer reduces the impedance to intensify the current to give abrasion.